The way I train is by taking the Pokémon I want to train and passing through each route of the game as the Pokémon rise in level, training by five levels per route as I do so. It works in quite a good way- at the beginning, my Sawsbuck, Artemis, was at Dragonspiral Tower and at level 40- still being EV trained against Basculin- and by mid-afternoon, Artemis was at the Giant Chasm, level 63 and having finished EV training. I would have stayed at Giant Chasm, but I wanted to train in the Double Grass, and as such, training a second Pokémon- my Druddigon, Discord- would be more effective.
Another way is to train against the Sports Arenas. I used that to level up my old ingame team.
Another bit of advice is to continuously fight against Morimoto, the Elite 4 and Cynthia whenever you can. I got said Sawsbuck from level 85 to 97 using these methods alone.